1. Field of the Invention:
The present invention relates to an apparatus for detecting intensity-modulated light signals which is composed of an optical detector comprising an optical electrical detector for the conversion of light intensity into corresponding, electrical power and comprising a load resistor connected to the detector, and composing a mixture stage into which a signal corresponding to the modulated intensity of the light signals to be detected and into which another signal synchronized with that signal and having a frequency defined by a modulation frequency of the one signal can be coupled, and from which a mixed signal formed of the one signal and of the other signal can be taken.
2. Description of the Prior Art
A frequent case in modern optics is that a light beam from the visible, the infrared or the ultraviolet range, that is intensity-modulated with a periodic oscillation, for example with a sinusoidal or square-wave oscillation, must be detected. This radiation can freely propagate or, on the other hand, can also be guided in a light waveguide. The concern is therefore to obtain a signal that is proportional to the amplitude of the intensity of fluctuation of the fundamental or, on the other hand, of a harmonic as well. A relatively small bandwidth is usually adequate for this signal, for example on the order of magnitude of 1 Hz. The frequency of the periodic oscillation usually lies on the order of magnitude of between 10 Hz and many MHz. The problem is to make the apparatus as sensitive as possible, i.e. the output signal proportional to the intensity fluctuation should contain, optimally, little noise, even given an extremely weak light signal.
The aforementioned problem occurs, for example, in fiber ring interferometers or ring resonators that are utilized as rotation sensors and also occurs in apparatus for documentation of absorption spectra, for example when measuring the attenuation of light wave guide links. Modulated light is frequently used for easier detectability.
The aforementioned problem has heretofore been solved in the following manner. The intensity-modulated light signal to be detected is fed to an optical detector device comprising an optical-electrical detector for converting light intensity into corresponding power and comprising a load resistor connected to the detector. The electrical signal circuit and a pre-amplifier must thereby have a great bandwidth such that the periodic oscillation impressed on the light beam can still be well amplified free of distortion. Subsequently, the electrical signal is supplied to a lock-in amplifier that contains a mixer or a multiplying stage and a low-pass filter, for example in the form of an integrator.
In addition to receiving the one electrical signal, the electrical mixer stage also receives another electrical signal that is supplied by an electronic oscillator and a fluctuating signal quantity oscillates corresponding to the frequency and phase of the intensity of the one signal that is modulated and to be detected. This is usually achieved by external synchronization.
A mixed signal formed of the one electrical signal and of the other electrical signal can be taken at an output of the mixer stage and the mixed signal, after filtering in the low-pass filter or integrator, corresponds to the amplitude of the modulated intensity or intensity fluctuation. The signal-to-noise ratio thereby becomes all the greater the longer the integration time.